Pressure swing adsorption is a well-known method for the separation of air to recover nitrogen-rich or oxygen-rich gas products. One particular application is the recovery of oxygen from air utilizing zeolite adsorbents to yield product gas containing up to 95 vol % oxygen. At this product purity, the remaining component in the product gas is essentially argon because argon and oxygen exhibit similar adsorption characteristics relative to nitrogen on typical commercially-available zeolite adsorbents. The adsorption selectivity of argon relative to oxygen is typically near or below unity for these adsorbents.
Recent work in the field of adsorptive air separation has shown that certain silver-exchanged zeolites, particularly silver-exchanged X-type zeolites, exhibit adsorption selectivity for argon relative to oxygen, which makes it feasible to produce oxygen at purities above 95 vol %.
The use of silver-exchanged zeolites in pressure swing adsorption processes for oxygen recovery has been reported by a number of workers in the field. U.S. Pat. No. 5,226,933 discloses a pressure swing adsorption process which uses a bed of silver mordenite for splitting oxygen from a gas comprising 95% oxygen and 5% argon to achieve an oxygen purity of at least about 99.7%. U.S. Pat. No. 5,470,378 describes a process for removing argon from a feed gas stream comprising oxygen and argon to yield a high purity oxygen stream. The process uses a bed comprising a Ag ion-exchanged type X zeolite wherein at least 80% of the available ion sites are occupied by silver.
Japanese Patent Application Kokai No. H10-152305 (Application No. H8-311051) discloses a pressure swing adsorption device for oxygen production which uses an adsorption column with a nitrogen adsorbent layer that adsorbs nitrogen selectively from a feed gas which contains nitrogen, oxygen, and argon. The nitrogen adsorbent layer contains at least a Ag ion-exchanged X-type zeolite. The adsorption column may contain more than one layer, and the layer on the gas exit side of the column contains a Ag ion exchanged X-type zeolite. It is claimed that argon is adsorbed more readily than oxygen and that a product gas containing 95% oxygen or higher can be obtained.
An apparatus for producing high purity oxygen from air is disclosed in European Patent publication EP 0 761 282 A2 wherein the apparatus comprises a pressure vessel in which is located a first bed of adsorbent which preferentially adsorbs nitrogen and spaced therefrom a second bed of adsorbent which preferentially adsorbs argon.
N. D. Hutson et al in an article entitled xe2x80x9cMixed Cation Zeolites: LixAgy-X as a Superior Adsorbent for Air Separationxe2x80x9d published in the AlChE Journal, April 1999, Vol. 45, No. 2, pp. 724-734 disclose the use of a silver-exchanged Lixe2x80x94Na-X zeolite for air separation. A simulation of a standard five-step PSA process was carried out using a single bed of Li94.2Na 0.7Ag 1.1-X zeolite having a Si/Al ratio of 1.0. The simulation used a feed gas of 22% oxygen and 78% nitrogen. An oxygen product purity of 96.42% was reported at an oxygen recovery of 62.74%. R. T Yang et al disclose similar simulation results in PCT International Publication No. WO 00/40332.
N. D. Hutson et al describe the properties of Lixe2x80x94Ag-X zeolites for air separation in an article entitled xe2x80x9cStructural Effects on Adsorption of Atmospheric Gases in Mixed Li,Ag-X-Zeolitexe2x80x9d published in the AlChE Journal, November 2000, Vol. 46, No. 11, pp. 2305-2317.
U.S. Pat. No. 4,880,443 discloses a series two-bed adsorption system for recovering oxygen from air in which nitrogen is selectively adsorbed in a first bed which contains a zeolite and argon is selectively adsorbed in a second bed which contains a carbon molecular sieve.
The production of high purity oxygen containing greater than 95 vol % oxygen from air by pressure swing adsorption with corresponding high oxygen recovery is an important objective in the industrial gas industry. The production of high purity oxygen at 97 vol % and above is particularly desirable for certain markets. The invention described below and defined by the claims which follow addresses this need with a multiple-zone pressure swing adsorption process which recovers oxygen at greater than 97% purity from air while achieving high adsorbent utilization by selective operation of the multiple-zone system.
The invention is a combined forward flow stage which is a part of a cyclic pressure swing adsorption process for the recovery of oxygen from a feed gas comprising oxygen, nitrogen, and argon. The combined forward flow stage comprises (a) passing the feed gas into a first adsorption zone containing an adsorbent selective for the adsorption of nitrogen over oxygen and argon, and withdrawing therefrom a nitrogen-depleted intermediate gas; (b) passing the nitrogen-depleted intermediate gas into a second adsorption zone containing an adsorbent which is selective for the adsorption of nitrogen over argon and selective for the adsorption of argon over oxygen; (c) withdrawing an oxygen-enriched product gas from the second adsorption zone; and (d) terminating the passing of feed gas into the first adsorption zone and withdrawing an oxygen-enriched depressurization gas from the second adsorption zone in the same flow direction as (c). Nitrogen breakthrough from the first adsorption zone occurs and nitrogen is adsorbed in the second adsorption zone after nitrogen breakthrough.
Nitrogen breakthrough from the first adsorption zone can occur during (a) and nitrogen can be adsorbed in the second adsorption zone after nitrogen breakthrough. Alternatively, nitrogen breakthrough from the first adsorption zone can occur during (d) and nitrogen can be adsorbed the second adsorption zone after nitrogen breakthrough.
The concentration of nitrogen in the nitrogen-depleted intermediate gas withdrawn from the first adsorption zone after nitrogen breakthrough can be between about 0.5 vol % and the nitrogen concentration in the feed gas entering the first adsorption zone. Preferably, the oxygen-enriched product gas contains at least 97 vol % oxygen.
The adsorbent in the first adsorption zone can comprise one or more adsorbents selected from the group consisting of NaX, CaX, CaA, LiNaKX, LiZnX, wherein X represents an X zeolite with a Si/Al ratio of between about 1.0 and about 1.25.
The adsorbent in the second adsorption zone preferably comprises a silver-exchanged X zeolite with a ratio of the argon Henry""s Law constant to the oxygen Henry""s Law constant at 23xc2x0 C. of at least about 1.05, and also which has a silver ion exchange level of less than or equal to about 0.7 of the total exchangeable sites in the zeolite. The cations in the X zeolite preferably comprise Li and Ag, and the ion exchange cation composition is of the form LixAgyMzX where 0.85xe2x89xa6x+yxe2x89xa61, 0.2xe2x89xa6yxe2x89xa60.7, and 0.0xe2x89xa6zxe2x89xa60.15.M represents one or more cations, and x, y, and z represent fractions of total exchangeable sites in the zeolite. The adsorbent can have a silicon/aluminum ratio of less than about 1.25 and an argon/oxygen selectivity of greater than about 1.05.
The feed gas preferably is air. Alternatively, the concentration of oxygen in the feed gas can be between about 20 and about 95%, and the concentration of argon in the feed gas can be between about 1 and about 5 vol%.
The volume occupied by the second adsorption zone can be greater than about 35% and less than 100% of the total volume occupied by the first and second adsorption zones. Preferably, the volume occupied by the second adsorption zone is greater than 50% and less than 100% of the total volume occupied by the first and second adsorption zones. More preferably, the volume occupied by the second adsorption zone is greater than 50% and less than or equal to about 75% of the total volume occupied by the first and second adsorption zones. The first and second adsorption zones can comprise individual layers of adsorbent in a single vessel.
In one embodiment, the feed gas can be provided by a gas product from another pressure swing adsorption process, and wherein the gas product contains up to about 95 vol % oxygen.
The invention also is an adsorber for use in a pressure swing adsorption process for the recovery of oxygen from a feed gas comprising oxygen, nitrogen, and argon. The adsorber comprises:
(a) a vessel having an inlet end and an outlet end;
(b) a first adsorption zone containing an adsorbent which is selective for the adsorption of nitrogen over oxygen and argon;
(c) a second adsorption zone disposed between the first adsorption zone and the outlet end, wherein the second adsorption zone contains an adsorbent which is selective for the adsorption of nitrogen over argon and selective for the adsorption of argon over oxygen, and wherein the volume occupied by the second adsorption zone is greater than 50% and less than 100% of the total volume occupied by the first and second adsorption zones;
(d) piping means at the inlet end for passing the feed gas into the adsorber; and
(e) piping means at the outlet end for withdrawing an oxygen product gas from the adsorber.
The first and second adsorption zones can comprise respectively first and second layers of adsorbent in the vessel. In the adsorber, the volume occupied by the second layer of adsorbent preferably is greater than 50% and less than or equal to about 75% of the total volume occupied by the first and second layers of adsorbent.
The adsorbent in the first layer of adsorbent can comprise one or more adsorbents selected from the group consisting of NaX, CaX, CaA, LiNaKX, LiZnX, wherein X represents an X zeolite with a Si/Al ratio of between about 1.0 and about 1.25. The adsorbent in the second layer preferably comprises a silver-exchanged X zeolite in which the ratio of the argon Henry""s Law constant to the oxygen Henry""s Law constant at 23xc2x0 C. is at least about 1.05 and in which the silver ion exchange level is less than or equal to about 0.7 of the total exchangeable sites in the zeolite. The adsorbent in the second layer preferably comprises an X zeolite having an ion exchange cation composition of the form LixAgyMzX where 0.85xe2x89xa6x+yxe2x89xa61, 0.2xe2x89xa6yxe2x89xa60.7, and 0.0xe2x89xa6zxe2x89xa60.15, with M representing one or more cations, and x, y, and z representing fractions of total exchangeable sites in the zeolite. Preferably, the adsorbent has a silicon/aluminum ratio of less than about 1.25 and an argon/oxygen selectivity of greater than about 1.05.